1. Field of the Invention
The present invention relates to an interference screw for endosteal fixation of a ligament and, more specifically, to an interference screw having a rounded back end and a larger than normal cannulation. The invention also relates to a cannulated sheath for an interference screw.
2. Description of the Related Art
When a ligament or tendon becomes detached from a bone, surgery is usually required to resecure the ligament or tendon. Often, a substitute ligament or graft is attached to the bone to facilitate regrowth and permanent attachment. Various methods of graft attachment are known, such as staples and sutures over buttons. However, such methods often do not provide a sufficiently strong attachment to withstand the normal tensile loads to which they are subjected.
A stronger graft attachment is obtained by using an interference screw to wedge a graft bone block to the wall of a graft tunnel formed through the bone. FIG. 1 illustrates this method, in which the graft 2, with bone blocks 4,6 at each end, is pulled through a graft tunnel 8 in the tibia 10, by applying a tensile force on sutures 12 attached to leading bone block 6. The leading bone block 6 is brought forward into the femur 14 until it is fully nested in a graft tunnel in the femur. Then, with tension applied to the graft 2 via sutures 12, a driver is used to insert interference screws 16 between the bone blocks 4,6 and the graft tunnel, first in the femur and then in the tibia, as shown in FIG. 2. Although interference screw attachment by the above-described method is more secure than using staples or the like, the graft can be inadvertently cut or frayed by the sharp edges of the interference screw during insertion and after fixation.
A thin guide pin is often used in conjunction with the interference screw to properly locate the screw against the bone block. Ideally, the screw, when fully nested, should be parallel to the graft. However, as shown in FIG. 10, during screw insertion with a thin guide pin, the thin pin tends to bend, allowing the screw to diverge off course and nest at an angle with respect to the bone block and the graft. Moreover, insertion of the guide pin is inaccurate, since the surgeon has to eye the correct depth of the insertion of the pin.